A variety of heat exchanger types are known. A common form, particularly for air or gas heat exchange with another gas or a liquid, is a fin-tube type exchanger. These are familiar in domestic heating and radiators, for example. In this type, one fluid flows through a tube, and the other fluid, typically a gas, flows essentially perpendicularly to the tube. Fins are attached to the tube to increase the area available for heat exchange, thereby minimizing the length of pipe and the associated pressure drop.
In seeking to further increase the rate of heat exchange from a tube, porous metal foams have been used as replacements for fins. These have the advantage of allowing flow of the outer fluid along the tube length, either counter-current or co-current as needed, and have an excellent ability to transfer heat. However, they are expensive, and can be difficult to bond firmly to a tube.